1. Field of the Invention
The present invention relates to a recording solution used for vaporization type heat transfer recording in which a recording solution is transferred to a recording material by vaporization.
2. Description of the Related Art
In recent years, as coloration of video cameras, computer graphics, etc. has progressed, coloration of not only monocolor recording but also hard copying has been increasingly needed. Therefore, color hard copy systems such as a dye diffusion type heat transfer recording system (sublimation type heat transfer recording system), a melting heat transfer recording system, an ink jet recording system, an electrophotographic recording system, a heat development silver salt recording system, etc. have been proposed. Of these recording systems, the dye diffusion type heat transfer recording system and the ink jet recording system can be used as a system for easily outputting a high-quality image by a simple apparatus.
In the dye diffusion type heat transfer recording system, an ink ribbon comprising an ink layer deposited on a base substrate and containing a high concentration of transfer dye dispersed in an appropriate binder, and a transfer material (for example, photographic paper or the like) having a dye receiving layer comprising a dyeing resin for receiving the dye are held between a platen roller and a thermal head, which are provided in a printer, so as to adhere to each other under predetermined pressure, and heat is supplied to the ink layer on the back side of the ink ribbon from the thermal head according to image information to thermally transfer the dye to the dye receiving layer from the ink layer. This transfer operation is repeated for image signals of each of yellow, magenta and cyan which are the subtractive primary colors, to obtain a full color image having continuous gradation. Therefore, the dye diffusion type heat transfer recording system facilitates miniaturization and maintenance of a recording system and has immediacy of image formation, and attracts attention as an excellent technique for obtaining high quality images equivalent to silver salt color photographs.
However, in the dye diffusion type heat transfer recording system, exclusive photographic paper is required for obtaining a high quality image, and large amounts of wastes are produced due to disposal of the ink ribbon. This system thus has the problem of increasing the running cost, and the disposal of large amounts of wastes is undesirable from the viewpoint of global environment. These problems are also caused in the melting heat transfer recording system.
The heat development silver salt recording system is capable of obtaining high-quality images, but has the problems of high running cost and high equipment cost due to the use of exclusive photographic paper and a disposable ink ribbon.
On the other hand, in the ink jet recording system, as disclosed in Japanese Patent Examined Publication Nos. 61-59911 and 5-217, droplets of a recording solution are flown from a nozzle provided in a recording head by a system such as a continuous vibration generating system (piezo system), a thermal system (bubble jet system) or the like, and adhered to the transfer material to recording an image. Since the ink jet recording system uses as the recording solution a solution which can be absorbed by plain paper, the system enables transfer to plain paper. The ink jet recording system produces substantially no waste and thus has the property of low running cost. Recently, particularly, the thermal system which can easily output color images has increasingly been popularized.
However, in principle, the ink jet system is difficult to obtain density gradation within a pixel, and is thus difficult to realize a high-quality image equivalent to a silver salt photograph within a short time, which can be achieved by the dye diffusion type heat transfer system.
Namely, in the conventional ink jet recording system, since one ink droplet ejected from the nozzle of an ink jet printer forms one pixel, and, in principle, it is difficult to obtain gradation in the pixel and impossible to form a high quality image. Although attempts have been made to obtain expression with pseudo-gradation by the dither method employing the high resolution of ink jet recording, image quality equivalent to that obtained by the sublimation type heat transfer system cannot be obtained, and the transfer speed is significantly decreased.
The electrophotographic system has low running cost and a high transfer speed, but has high equipment cost.
As described above, in present conditions, the conventional recording systems cannot simultaneously satisfy all requirements such as image quality, running cost, equipment cost, transfer time, etc.
As a recording system which simultaneously satisfies these requirements, a non-contact type heat transfer recording system such as a vaporization type (or a dye vaporization type) heat transfer recording system has recently attracted attention in which a recording agent such as a dye itself or a recording solution is vaporized or atomized and transferred to a recording material such as photographic paper or the like (EP 94101201. 5).
In a printer head used in this recording system, a liquid recording agent, e.g., a melted dye or a recording solution obtained by dissolving a dye in a solvent, is introduced into a vaporization portion having a porous structure by the capillary phenomenon, and then held therein. Heat is selectively applied to the liquid recording agent held in the vaporization portion by heating means such as a laser or the like according to recording information to vaporize the recording agent or generate a mist having a diameter of 1 .mu.m or less. The vaporized recording agent or mist is transferred to the photographic paper positioned opposite to the vaporization portion with a gap of 10 to 300 .mu.m therebetween. Since the vaporization portion has a porous structure, the surface area of the vaporization portion (heating portion) is increased, and thus the liquid recording agent can be constantly supplied to the vaporization portion by the capillary phenomenon and held therein.
Therefore, in the vaporization type heat transfer recording system, small droplets of the recording agent can be formed, as compared with a known ink jet system. Further, since the number of the droplets of the recording agent can freely be controlled by changing the heating energy applied to the recording material according to recording information, multivalue density gradation is possible, and an image (for example, a full color image) with quality equivalent to or higher than that of an image obtained by the silver salt system can be recorded.
In addition, in the vaporization type heat transfer recording system, although the recording agent is vaporized or atomized, the ink layer of the ink ribbon need not be heated, and the ink ribbon and the recording material need not be pressed by high pressure, unlike the conventional sublimation type heat transfer recording system. This point is advantageous for decreasing the size and weight of the printer. Also, since the portion (the vaporization portion or heating portion) for vaporizing the recording agent does not contact the recorded material, no heat fusion occurs therebetween, and recording is possible even if the dye and the dye receiving layer region of the recorded material have low compatibility. There is thus the advantage of widening the ranges of design and selection of the dye and the dye receiving layer resin.
As the recording agent applied to this vaporization type heat transfer recording system, a dye itself or a recording solution obtained by mixing a dye and an appropriate solvent can be used, as described above. However, the recording solution has excellent handling properties, and is thus preferably used.
A solution which can be used as the recording solution is one having an appropriate vaporization rate and abrasion rate and sufficient heat resistance, and exhibits a flowing state at 200.degree. C. or less singly or in a mixture with another third component. Specifically, disperse dyes, oil-soluble dyes, basic dyes and acid dyes can be used as the dye, and dibutylphthalic acid, toluene, and ethylnaphthalene can be used as the solvent.
On the other hand, photographic paper suitably used for the vaporization type heat transfer recording system is paper having a transfer surface which need not have high compatibility with the transfer dye but at least has appropriate compatibility therewith, and capable of easily receiving the transfer dye to accelerate development of the original color of the dye, and fixing the dye. For example, in use of a disperse dye, paper having a surface coated with a polyester resin, polyvinyl chloride resins, or acetate resin can preferably used as the photographic paper.
In fixing the dye transferred to the photographic paper, the transferred image may be heated to diffuse the transferred dye on the surface of the paper into the dye receiving layer.
As described above, like the conventional heat transfer recording system, the vaporization type heat transfer recording system enables a reduction in the size of the printer and facilitates maintenance thereof. Also a recorded image can immediately be obtained by the heat transfer operation, and the image obtained has high quality and good gradation.
However, for conventional recording solutions used for the vaporization type heat transfer recording system, no consideration is given to the relative degree of vaporization between the solvent and the dye, and the solvent and the dye are generally combined so that the solvent is more easily vaporized than the dye. Therefore, in a recording operation, i.e., in the process of vaporizing the recording solution, the dye concentration of the recording solution is increased, thereby causing the problem of producing precipitates of the dye. Accordingly, there occurs the problems of deterioration in the transfer efficiency, burning in the vaporization portion due to carbonization of the dye precipitates, etc.
In order to solve these problems, it is considered to form a recording solution by using a dye and a solvent which have substantially the same boiling point or degree of vaporization at the same temperature. However, it is very difficult to find a solvent which has a melting point of room temperature or less, preferably 0.degree. C. or less, and a boiling point within the boiling point range of 300 to 450.degree. C. of usual dyes, and which can sufficiently dissolve the dye.